One of the current methods used to make fiberglass reinforced plastic (FRP) articles is the pultrusion process. The pultrusion process coats or saturates continuous glass fibers or other reinforcement materials and pulls them through a die to form a continuously molded linear part. The glass fibers or other fiber reinforcement within the pultruded FRP material provide good durability, strength and surface properties required of a structural panel. After molding, the part can be sawed off or cut to desired lengths to produce discrete FRP articles. The resins typically used in pultrusion are polyester resins, although other resins such as phenolic, epoxy, or vinylester resins can be used.
The pultrusion processes can be used to make lineal members having varied cross sections. For example, pultrusion can be used to produce vinyl window lineals having complex cross sectional shapes. Another use of pultrusion is to form building panels or structural panels for use in the construction industry. Such panels are often used in transportation facilities, such as airport passageways and subway tunnels.
Some pultrusion products are used for building materials in settings where they are exposed to human beings. Preferably, these products are made from materials which reduce smoke generation in the event of a fire. Such building materials would have an important property of low smoke generation. Phenolic resin is known to have superior properties in reducing smoke generation in fire situations.
In some cases, it is desirable to make a sandwich structural panel having a pultruded FRP exterior and a core material which provides insulation, structural and other properties. A sandwich panel is a combination of thin, high-strength facings on each side of a much thicker, lightweight core material. To that end, some manufacturers of structural panels have taken a hollow pultruded FRP panel and filled the panel with a foamable mixture to create a foam-filled or foam core structural panel having an FRP pultruded skin. Typically, such a foaming process involves a urethane foam.
One of the inherent problems with a post-pultrusion foaming process is that the bond between the foam and the FRP skin is inherently weak, and the entire panel lacks sufficient integrity and strength for most composite building material requirements. A crucial element of a successful sandwich panel is a tough bond between the facing and the core.
Attempts to pultrude an FRP skin onto a preformed foam core board have been largely ineffective. Such attempts have usually employed a polyester resin, which does not have low smoke generation characteristics. One of the major problems of such attempts has been the friability of the foam material when subjected to the pressure of the pultrusion process. Without a significant amount of pressure, the FRP skin will not bond to the foam material. However, the significant pressure required for good bonding of the FRP skin to the foam tends to fracture and otherwise degrade the foam core. Further, the urethane foam materials heretofore used do not provide very good smoke properties during a fire.
Most previous attempts to make FRP skin/foam core sandwich panels were batch processes rather than continuous flow-through pultrusion processes. As a result of the batch-oriented nature of the processes, quality control during large-scale production tended to be very difficult. Such structural panels tended to display significant variations in their mechanical, electrical and surface properties, both from one production run to the next and even within the same production run.
In view of the deficiencies of existing methods for making composite sandwich structural panels, there is a need for a method which can consistently make good quality insulated FRP construction panels having low smoke-generating properties.